This invention relates to roller cutter drill bits and more particularly to improved means for retaining roller cutters on a rotary drill bit.
Heretofore, various means have been employed for retaining roller cutters on a drill bit. For example, ball bearings have been mounted in opposed annular grooves or raceways of the roller cutter and associated cylindrical journal on which the roller cutter is mounted for rotation. A disadvantage of having ball bearings for retainers is that one of the ball bearings or adjacent raceways may fail with metal fragments or particles entering the friction bearing for damaging such bearing as well as the seal ring thereby resulting in lubricant loss and possible subsequent bearing failure. Also, metal spalling occurs from the small almost point bearing contact of the balls with adjacent surfaces resulting in fatigue and possible chipping of the balls. With substantial wear on the balls, it is possible that the roller cutter may be released from its journal.
Another means for retaining a roller cutter on the cylindrical journal of a drill bit is illustrated in U.S. Pat. No. 4,236,764 dated Dec. 2, 1980 in which a snap ring is positioned within an annular groove of the cylindrical journal and is urged radially outwardly into a registering groove on the roller cutter. However, the snap ring retainer provide a relatively small contact area with the adjacent surfaces. Further, the cross-sectional area of a snap ring is limited in area which may be undesirable, particularly for large diameter roller cutters, and could possibly result in a roller cutter being lost from its journal. Also, the snap ring is oftentimes difficult to assemble requiring fixtures for assembly and possible bearing damage during the assembly process.
Another example of means to retain roller cutters is illustrated in U.S. Pat. No. 3,971,600 dated July 27, 1976 in which a generally rectangular threaded retainer ring is utilized for retaining a roller cutter onto a cylindrical journal of a drill bit. The threaded retainer ring permits a large bearing contact area for retaining the roller cutter and the rectangular cross-sectional area of the threaded retainer ring is relatively large so that the loss of a roller cutter from its bearing journal is minimized. The threaded retainer ring is formed of two semicircular portions mounted in an annular groove in the cylindrical bearing with circumferential external screw threads engaging mating internal screw threads on the roller cutter. The retainer ring is held against rotation as the roller cutter is threaded onto the ring.